Hundreds of thousands of Catalans prepared to rally in the streets of Barcelona today in what campaigners hope will be a show of support for independence after Madrid moved to block a planned referendum on the region's split from Spain.

The 'Diada' day of Sept. 11, which commemorates the fall of Barcelona to Spain in 1714, is often used by activists to voice their demands for an independent state. Coachloads of demonstrators travel to Barcelona from villages in the region.

Hostility between Madrid and Barcelona has ramped up since Spain's Constitutional Court last Thursday suspended the referendum, planned for Oct. 1, following a legal challenge by Prime Minister Mariano Rajoy. The Madrid government says the referendum contravenes the constitution, which states that Spain is indivisible.

Under Article 155 of Spain's constitution, Madrid has the power to intervene directly in the running of Catalonia's regional government, forcing it to drop the vote. This could involve sending in the police or suspending the regional government's authority to rule.

Spain's state prosecutor has begun criminal proceedings against local president Carles Puigdemont and 13 members of his cabinet on charges of misuse of public money, disobedience and abuse of office after the Catalan parliament approved the referendum last Wednesday. Rajoy has urged Catalan civil servants and mayors to uphold the law and to stop any preparations for the referendum over the next three weeks.

In February, the Constitutional Court ruled against the referendum and warned Catalan leaders they faced repercussions if they continued with their project.

In 2014, Catalonia held a non-binding vote under then President Artur Mas, in which more than 80 percent of those who cast a ballot chose independence, although just 2.3 million out of 6.3 million eligible voters took part, but in holding the symbolic referendum, Mas went against Spain's Constitutional Court, which had outlawed the vote, even if it was non-binding. He was later put on trial and banned from holding office for two years.

At the height of pro-independence fervor in 2012, during a deep economic recession in Spain, around one million people took to the streets waving the Catalan flag and singing the Catalan anthem. Polls have shown support for independence waning since then, and those wanting a separate state are in a minority. However, a majority of Catalans want to hold a referendum on the issue.

The Catalonia region centered on Barcelona generates a fifth of Spain's gross domestic product. It self-governs in several important areas, such as police, health and education. But key areas such as taxes, foreign affairs and most infrastructures are in the hands of the Spanish government. Both Catalan and Spanish are spoken in the region of 7.5 million people.